1. Field of the Invention
The present invention relates to a system and method for improving the operation of a boiler.
2. Description of the Background
FIG. 1 shows a conventional boiler system that includes two sets of fans, an “A” set and a “B” set. The A-side set of fans includes forced draft fan 101a, which includes a silencer 105a for reducing noise. The forced draft fan 101a forces combustion air through combustion air conduit 109. Combustion air conduit 109 includes duct sections 111a, 111b, 111c, 111d, 111e, and 111f, which are shown as solid lines with the arrows showing the direction in which combustion air from the forced draft fan 101a is forced. The combustion air conduit 109 also includes shutoff damper 113a, which can be used to reduce or shut off completely the flow of combustion air between duct sections 111a and 111b. 
Air preheater 115a is located between duct sections 111b and 111c. Air preheater 115a heats combustion air flowing from fan 101a before the air reaches duct section 111c. 
Duct sections 111d and 111e branch from duct section 111c. Duct section 111d directs combustion air into the A-side air heater 119a. Air heater 119a is divided into two parts, as shown conceptually, by the dashed line in FIG. 1. Air heater 119a may be any suitable device for heating combustion air, such as a CE LJUNGSTROM regenerative air preheater, for example. An exemplary air heater 119a may include a large disk of stacked steel baskets that rotate from the hot side of the air heater 119a toward the cold side of the air heater 119a. The hot side of the air heater 119a (conceptually shown on the left of the dashed line bisecting air heater 119a in FIG. 1) receives hot flue gas. The cold side of air heater 119a (shown on the right of the dashed line bisecting air heater 119a in FIG. 1) receives combustion air via the duct section 111d of the combustion air conduit 109. Air heater 119b a heats the combustion air 119a it receives from duct section 111d. The combustion air heated by air heater 119a is forced out of air heater 119a and is directed by duct section 111f if to the boiler 121, which bums a mixture of combustion air received from duct section 111f and fuel. The burning of fuel and combustion air generates hot flue gas, which is directed away from boiler 121 by flue gas conduit 129, shown as a dashed line. Flue gas conduit 129 includes duct section 123a which directs flue gas from boiler 121 to the hot side of the A-side air heater 119a. Flue gas conduit 129 also includes duct section 123b for directing flue gas away from the air heater 119a, duct sections 123c and 123j, which branch from duct section 123b, shutoff damper 127a for controlling the volume of flue gas flowing between duct sections 123c and 123d, duct section 123d for directing the flue gas to an A-side induced flow fan 131a; duct section 123e for directing flue gas away from induced draft fan 131a, shutoff damper 133a; for controlling the volume of flue gas flowing between duct sections 123c and 123f and duct sections 123f and 123g for directing flue gas toward the stack 135.
The A-side induced draft fan 131a pulls flue gas away from the boiler 121 in the direction shown by the arrows along flue gas conduit 129. Induced draft fan 131a may be implemented by any suitable fan for that purpose. Shutoff dampers 127a and 133a perform the same or similar function as shutoff damper 113a. Shutoff dampers 115a, 127a, and 133a may be implemented by any suitable device for shutting off, opening, and varying the flow of air and/or gases through conduit.
The B-side of the boiler system in FIG. 1 includes B-side forced draft fan 101b, the silencer 105b of the forced draft fan 101b, air preheater 115b, B-side air heater 119b, B-side induced draft fan 131b, and shutoff dampers 113b, 127b, and 133b. These B-side components may be implemented in the same manner as the corresponding A-side components and perform the same or similar function for the B-side as the A-side components perform for the A-side.
Combustion air conduit 109 includes duct section 111g for directing the flow of combustion air from the fan 101b to the shutoff damper 113b, duct section 111h for directing combustion air from the shutoff damper 113b to the air preheater 115b, duct section 111i for directing air away from the air preheater 115b, duct sections 111k and 111j which branch from duct section 111i to duct section 111e and to air heater 119b respectively, and duct section 111l which directs combustion air from the air heater 119b to the boiler 121. Together, the duct sections 111e and 111k form combustion air crossover conduit, which allows combustion air to flow between the A-side and the B-side of the boiler system.
Flue gas conduit 129 includes duct section 123h for directing flue gas from the boiler 121 to the hot side of the air heater 119b (conceptually shown as the portion of the air heater 119b on the right side of the dotted line that bisects the air heater 119b), duct section 123i for directing flue gas from the air heater 119b, duct section 123k which branches from duct section 123i and directs flue gas toward duct section 123j, duct section 123l which branches from duct section 123i and directs flue gas toward the shutoff damper 127b, duct section 123m for directing flue gas from the shutoff damper 127b to the induced air fan 131b, duct section 123n for directing flue gas from the fan 131b to the shutoff damper 133b, and duct section 123o for directing flue gas to the duct section 123g and eventually on to the stack 135. The shutoff dampers 113b, 127b, and 133b may also be considered part of the flue gas conduit 129 and regulate flow in the same manner as the corresponding counterpart shutoff dampers on the A-side (i.e., shutoff dampers 113a, 127a, and 133a). The duct sections 123j and 123k form flue gas crossover conduit for permitting the flow of flue gas between the A-side and the B-side of the boiler system in FIG. 1.
During normal operation, the forced draft fans 101a and 101b push combustion air through the cold sides of the air heaters 119a and 119b and toward the boiler 121. The induced draft fans 131a and 131b pull flue gas away from the boiler through the hot sides of the air heaters 119a and 119b, and toward the stack 135. A problem with the operation of this and other boiler systems is that acid, such as sulphuric acid, condenses in the flue gas. In this respect, the acid dew point is important because when the flue gas and components in contact with the flue gas have a temperature below the acid dew point, acid condenses out of the flue gas. Condensed acid corrodes the components of the boiler system that it contacts and also increases the opacity of the flue gas. If the opacity is regulated (by state or federal agencies, for example), then it is normally desirable to decrease the opacity of the flue gas.
As the boiler system starts up or shuts down, the flue gas temperature in the air heaters 119a and 119b as well as the flue gas output ductwork temperatures (i.e., the portions of the flue gas conduit 129 downstream of the air heaters 119a and 119b and the induced draft fans 131a and 131b) are below the sulphuric acid dew point. As noted, this allows sulphuric acid to condense and collect on the colder surfaces. During startup and initial unit loading, this condensed sulphuric acid is re-volatized when flue gas temperature rises to approximately 240°. At this temperature, the re-volatized sulphuric acid is a combination of fine droplets and gas. The fine droplets cause elevated opacity in the stack 135. As the temperature continues to rise above the acid dew point (approximately 270° F.) all of the sulphuric acid becomes gaseous and no longer contributes to opacity.
Several solutions for reducing sulphuric acid mist have been proposed. One such proposal is to use a chemical additive that reduces the formation of sulphuric acid. These additives, such as magnesium-oxide, are injected into the flue gas during startup and shutdown of the boiler system. This approach has several problems, not the least of which is that the use of additives may pose environmental risks and/or be prohibited by law.
Another approach is to increase the temperature of the combustion air and/or the flue gas during startup and shutdown. These techniques attempt to reduce the effect of the ambient temperature on the time it takes to warm the flue gas above the acid dew point. An example of one such system is described in U.S. Pat. No. 4,932,464, which is incorporated herein by reference in its entirety. Other techniques for increasing flue gas temperature involve air heater bypass. Air heater bypass may be implemented, for example, by causing combustion air to flow around, rather than through, an air heater. For example, extra ductwork could be added to the boiler system of FIG. 1 to cause combustion air to flow directly from duct section 111d to 111f and from duct section 111j to duct section 111l, without passing through either of the air heaters 119a and 119b. Shutoff dampers may be used along this extra duct work to vary the amount of bypass. Air heater bypass causes the air heaters to increase in temperature as a result of receiving less of the combustion air, which is relatively cooler than the hot flue gasses received on the hot sides of the air heaters. This solution, however, is costly because it requires extra ductwork to be able to control the bypass of air flow around the air heaters. Additionally, it may not be feasible, or even possible, to retrofit existing boiler systems with the ductwork required to enable air heater bypass.